1. Field of the Invention
The present invention relates to a method and apparatus for reading images formed of material using a line sensor and, more particularly, to a process of producing an image signal.
2. Description of the Related Art
In image readers used with equipment such as a facsimile, an image scanner, a digital copier, or the like, the image sensor is composed of a one-dimensional array of photoelectric conversion elements. The image sensor receives reflected light from the document or the document illuminated by a light source, and converts the light into an electric signal representing the image of the document.
There are two types of image readers. In the first type, the document is imaged reduced onto a CCD (charge coupled device) image sensor by a spherical lens. In the second type, the document is imaged onto an image sensor composed of an array of photoelectric conversion elements the length of which is equal in to the width of the document, with a magnification of 1:1 by equimultiple focusing optics such as a rod lens array. The second type of image reader is called a contact type image sensor.
FIG. 8 is a cross sectional view illustrating one example of the contact type image sensor. An LED (light emitting diode) array 81 has a condenser rod lens disposed thereover, which is formed slightly longer than the reading width of a document. The LED array 81 is fixed to the slant surface of a supporting member 85 so as to illuminate a document 82 at an angle of almost 45 degrees. A rod lens array 84 and an image sensor 86 are disposed under reading position 83 of document 82. The image sensor 84 is formed of an array of photoelectric conversion elements, which are mounted on supporting member 85. In rod lens array 84, a great number of so-called refractive-index distributed rod lenses, in which the refractive index progressively diminishes from center to circumference, are arrayed with the optical axes directed top to bottom in the drawing and parallel to each other. The rod lenses have focal points located at points which are symmetric with respect to the lengthwise midpoint of the array. Document 82 and image sensor 86 face each other at the focal points of the rod lenses. That is, a ray of light emitted from LED array 81 and traveling in the direction of an arrow indicated is transmitted through rod lens array 84 after reflection from the document and then imaged onto the photoelectric conversion elements of image sensor 86.
Image sensor 86 is electrically connected to a circuit board 87 via a terminal 88. An electronic circuit including a driving circuit for driving and controlling image sensor 86, a circuit for processing signals resulting from photoelectric conversion in image sensor 86, and a connector 89 adapted for signal communication with, for example, facsimile incorporating the image reader are mounted on circuit board 87.
There are various types of image sensors. The thin film type uses a line sensor which is produced by a thin film technique and is longer than the width of a row to be scanned of the document. The multichip type in has a plurality of IC line sensors which are arrayed to cover the scanning length. FIG. 9 is a plan view of one example of the multichip type image sensor. In this example, four CCD sensors (line sensors) 91, 92, 93 and 94 are arrayed in a staggered fashion.
One of properties of the line sensor described above is that its output characteristics vary with ambient temperature. That is, even if the amount of incident light remains unchanged, the output signal level varies as ambient temperature varies. It is thus required to carry out compensation for output-level errors due to variation in ambient temperature, the so-called dark signal compensation.
For this reason, as shown in FIG. 9, end portions 91a to 94a of CCD sensors 91 to 94, which are on the signal reading start position side, are shaded by a metal film to make light-shielded pixels (about ten pixels in general) for each sensor, which produce optical black reference signals for the dark signal compensation. That is, the amount of light incident on the light-shielded pixels is zero, and their output signals (light-shielded signal) represent pure black in the state under the influence of ambient temperature. Hence, differential amplification of an output signal (an effective signal) output from a nonshaded photoelectric conversion element (an effective pixel) and the light-shielded signal will produce a signal free from the influence of variation in ambient temperature.
However, there are various problems with the conventional image reader, as described below.
(1) Since the black reference signal output from the light-shielded pixel in the end portion of a line sensor is used for compensating the signals output from all the effective pixels in the line sensor, a sample and hold circuit must be used to hold the black reference signal for a scanning interval of the line sensor. With the sample and hold circuit, however, there is a possibility of reduction of its holding voltage with time due to the leakage of a capacitor, the insulation performance of the circuit board and the characteristics of an operational amplifier. If the holding voltage is reduced, dark signal compensation will not be performed accurately.
(2) Line sensors such as CCD sensors may differ from each other in direct-current offset voltage of output signal. With the above multichip type of image reader, therefore, the direct-current offset voltages may vary in part in the output image signal, and the image reader will not produce an accurate image signal.
(3) In such a multichip type of image reader as shown in FIG. 9, CCD line sensors 91 to 94 are sequentially driven in such a way that when the scanning operation of one CCD sensor is completed, the next CCD sensor starts scanning. Hence, if light-shielded pixels are provided to obtain the black reference signals for the dark signal compensation as described above, non-signal portions will be produced in effective pixel signals in serializing the effective pixel signals output from effective pixels of the line sensors, causing inconvenience to treatment of signals.
(4) Output signals from the light-shielded pixels in each of line sensors are sequentially entered into and held by a sample and hold circuit. That is, the signal used as a black reference signal is the signal which is at last applied to the sample and hold circuit within the signals output from a plurality of light-shielded pixels respectively. However, photoelectric conversion elements constituting the same line sensor may produce slightly varying outputs because of variations in their performance though they are shaded. Hence, the signal output from one of light-shielded pixels is not necessarily suited for the black reference signal.
As described above, the conventional image reader suffers from a disadvantage in that normal image signals cannot be obtained because of various causes.